The present invention related to a lifting device for displacing a stack of goods, a pack, or a load, comprising:
a substantially horizontally extending support forming a support surface, comprising at least two support elements each moveable in a first respective vertical plane, such as, for instance, two prongs of a fork, for supporting the stack during picking up, transporting, and putting down of the stack, the support elements during picking up being slid under the stack at one of the sides of the stack and during putting down being pulled from under the stack at said side;
a lifting member, movable connected to the support elements and displaceable in a second vertical plane, and
a connecting device for movably connecting the support elements to the lifting member.
For transporting sorted products such as eggs, vegetables or fruits, trays are typically used. These trays are manufactured from flexible material so as not to damage these products. Moreover, with this material, any desired shape can be provided very suitably. The channels thus provided therein between the nests and the edges at the lateral sides also enable stacking such trays in an advantageous manner, there being generally formed stacks of six trays. Moreover, the prongs can readily be passed into the channels in order to lift one or more trays.
A drawback is that situations may occur in which less regular towers of stacks are formed, i.e. towers which are not entirely vertical. This may occur both during stacking as well as after some time, when, for instance, moisture may cause parts of the trays to sag.
These circumstances render it problematic to pick up such stacks and also to transfer them on top of each other. This last may, for instance, be the case during the loading of pallets.
The use of forks for transporting stacks is generally known. More in particular, the displacement of obliquely positioned stacks, especially of loaded pallets, is known from DE 2935553. In this document, a fork lifting device is shown of which each of the prongs which compose a fork and are fixedly horizontally directed, is insertable in such a manner that obliquely positioned pallets can also be picked up and put down, obliquely if necessary. To that end, each prong-bearing arm is connected to a corresponding vertical lifting guide in such a manner that over a well-defined vertical range, generally not too large, the insertion height will, during insertion, be assumed in an advantageous manner without obstruction. In particular, the hook-shaped construction of prong and arm is coupled to said lifting guide according to a parallelogram connection. Moreover, there is provided a biasing force which, during displacement of a prong from its stop position or rest position, drives back this loaded prong slightly and, in unloaded condition, largely compensates the weight of the construction itself. In this manner, in the case of obliquely positioned pallets, the prongs, inserted at staggered heights, are for instance pushed against one of the layers of the pallet platform, after which the lifting can take place in a reliable manner.
Such prongs fall short for picking up and putting down trays. During picking up and putting down stacks of trays respectively from and on stacks of trays disposed therebelow not entirely vertically, the prongs, which remain horizontal and are suitable for displacing constructions which remain substantially rigid, such as pallets, will lead to unstable insertion, sliding off and pulling away of the prongs. Due to this, stacks may get out of plumb and, consequently, even fall over.
To overcome this problem during picking up or putting down trays, the device according to the invention is characterized in that the connecting device comprises:
first members for, during picking up or putting down of the stack, moving a support element freely relative to the lifting member through an at least small distance relative to the lifting member in substantially vertical, upward direction;
second members for freely rotating a support element through an at least small angle about an axis perpendicular to said first respective vertical plane, the second members allowing free rotation only when, during picking up or putting down of the stack, the support element has been moved in said upward direction.
With such device, it is advantageously achieved that during picking up or putting down a stack from or on top of a stack which is positioned not entirely perpendicularly, one or more support elements or prongs abut against the support surface then present, so that no forced pushing of stacks or insertion or pulling away of prongs takes place and new towers of stacks do not become further unstable thereby. A further advantage is that in the case of trays filled with fragile products such as eggs, fruits or vegetables, they are not damaged.
In a further exemplary embodiment of the present invention, the lifting device is characterized in that the members comprise two sliding pins, arranged one above the other, and a sliding slot, such that during picking up or putting down of the stack, the movements of the pins are guided in the sliding slot, while for movements in upward direction there is provided a straight slot, and for rotation there is formed a slot which slightly diverges upwards.
With this combination of pins and slot, a free movement both in upward direction and in direction of rotation is readily effected. During picking up stacks of trays, the prongs will easily be able to adjust themselves to the channels, while during putting down of a stack, the support elements or prongs can be pulled away without resistance, whereupon they will slide back into the rest position or stop position by their own weight.